Phyllosticta Leaf Spot

On maple leaves you'll see discrete, angular brown or black spots appearing by late spring or early summer, concentrated on older foliage first. These spots have well-defined margins and don't typically form the explosive merging pattern seen in some other leaf spots. Wet weather in spring and early summer accelerates symptom development, and overhead watering creates ideal conditions for the fungus to spread through splash and spore dispersal.

Phyllosticta leaf spot on maples is rarely life-threatening but causes premature defoliation that weakens the tree and reduces its vigor. The fungus overwinters on fallen leaves, so your most effective strategy is sanitation: rake and destroy all fallen leaves in autumn before spring spore release. Avoid overhead irrigation whenever possible, and if you must irrigate, water early in the morning so foliage dries quickly. Good air circulation through pruning helps tremendously in the humid PNW.